How to define different dependencies for different product flavors

To define a flavor specific dependency you can use proCompile instead of compile in your dependency section. When you run gradle properties you get an overview of automatic created configurations.

The correct build file looks like this:

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.2.3'
    }
}
apply plugin: 'com.android.application'

repositories {
    mavenCentral()
}

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"

    defaultConfig {
        minSdkVersion 10
        targetSdkVersion 22
    }

    productFlavors {
        pro {
            packageName "de.janusz.journeyman.zinsrechner.pro"
        }
        free { }
    }
}

dependencies {
    compile 'com.android.support:support-v4:22.2.0'
    freeCompile 'com.google.android.gms:play-services-ads:7.5.0'
}